[Solved] Oracle 12.1 performance on NVL and Left Outer Join

How to optimize this SQL query?

In case you have your own slow SQL query, you can optimize it automatically here.

For the query above, the following recommendations will be helpful as part of the SQL tuning process.
You'll find 3 sections below:

  1. Description of the steps you can take to speed up the query.
  2. The optimal indexes for this query, which you can copy and create in your database.
  3. An automatically re-written query you can copy and execute in your database.
The optimization process and recommendations:
  1. Avoid Calling Functions With Indexed Columns (query line: 6): When a function is used directly on an indexed column, the database's optimizer won’t be able to use the index. For example, if the column `date_col_1` is indexed, the index won’t be used as it’s wrapped with the function `NVL`. If you can’t find an alternative condition that won’t use a function call, a possible solution is to store the required value in a new indexed column.
  2. Avoid Calling Functions With Indexed Columns (query line: 6): When a function is used directly on an indexed column, the database's optimizer won’t be able to use the index. For example, if the column `date_col_2` is indexed, the index won’t be used as it’s wrapped with the function `NVL`. If you can’t find an alternative condition that won’t use a function call, a possible solution is to store the required value in a new indexed column.
  3. Avoid Selecting Unnecessary Columns (query line: 2): Avoid selecting all columns with the '*' wildcard, unless you intend to use them all. Selecting redundant columns may result in unnecessary performance degradation.
The optimized query:
SELECT
        * 
    FROM
        table_1 
    WHERE
        NVL(table_1.date_col_1, table_1.date_col_2) <= table2.date_col_3(+)

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* original question posted on StackOverflow here.